The Network Working Group has produced a number of X.509 related specifications as listed below:
RFC 6151 - Updated Security Considerations for the MD5 Message-Digest and the HMAC-MD5 Algorithms
RFC 5297 - Synthetic Initialization Vector (SIV) Authenticated Encryption Using the Advanced Encryption Standard (AES)
RFC 5217 - Memorandum for Multi-Domain Public Key Infrastructure Interoperability
RFC 5084 - Using AES-CCM and AES-GCM Authenticated Encryption in the Cryptographic Message Syntax (CMS)
RFC 5114 - Additional Diffie-Hellman Groups for Use with IETF Standards
RFC 4231 - Identifiers and Test Vectors for HMAC-SHA-224, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512
RFC 3923 - End-to-End Signing and Object Encryption for the Extensible Messaging and Presence Protocol (XMPP)
RFC 3565 - Use of the Advanced Encryption Standard (AES) Encryption Algorithm in Cryptographic Message Syntax (CMS)
RFC 3447 - Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1
RFC 3394 - Advanced Encryption Standard (AES) Key Wrap Algorithm
RFC 3217 - Triple-DES and RC2 Key Wrapping
RFC 3125 - Electronic Signature Policies
RFC 2631 - Diffie-Hellman Key Agreement Method
RFC 2104 - HMAC: Keyed-Hashing for Message Authentication.
This RFC has a list of errata list of errata.